Secondary batteries have a high potential for a variety of fields of application. For example, secondary batteries based on the lithium-sulfur redox pair may be used, in particular based on the high specific capacity of sulfur. For a number of applications, however, there may still be potential for improvement, in particular of the cycle stability of such energy stores.
It is thus known that lithium-sulfur batteries, for example, have a so-called shuttle mechanism under some circumstances. This mechanism is based essentially on differences in solubility of the resulting lithium-sulfur species. The overall reaction Li+S8↔Li2S taking place in such batteries specifically includes multiple polysulfide intermediates having a sulfur chain length between three and eight. These are readily soluble in conventional electrolyte systems. However, the Li2S2 and Li2S reaction products are almost insoluble in many solvents and electrolyte systems.
Based on the difference in solubility of the polysulfides, long-chain polysulfides diffuse to the metallic lithium anode, where they react directly with lithium and are reduced. The resulting medium-chain and short-chain polysulfides may in turn diffuse to the cathode, where they are oxidized to longer-chain polysulfides or may enter into a comproportionation reaction with any present sulfur species of a higher sulfur oxidation level. Insoluble sulfide species, which may precipitate at the anode, may be formed during reaction with lithium at the anode. A fast polysulfide shuttle may also complicate or prevent complete recharging of the cell.
U.S. Pat. No. 7,282,296 B1 discusses separators and electrode structures for protecting active metal anodes from harmful reactions with air, moisture or other battery components in particular. The separator should have a high degree of ionic conductivity to simplify its manufacture and to improve the efficiency of the battery. The separator has two layers of differing chemical compatibility. The first layer is in direct contact with the anode and is chemically compatible with the anode. It therefore does not form with another component of the battery, the surroundings or the anode itself a compound that would be harmful for the battery. The second layer is essentially impermeable for additional components of the battery, for instance the electrolyte and the surroundings, and thus prevents contact between the battery and these components. The second layer is chemically compatible with the first layer.